This invention relates to telephone switching equipment and, in particular, to line concentrators, which provide telephone connections between the system central office unit and a number of subscribers over a lesser number of trunk lines extending from the central office unit.
Line concentrator technology evolved first in the 1960's. In general terms, a concentrator incorporates a central office unit located in the main telephone switching central office, which, in effect, replaces the normal direct subscriber connections. There is no identifiable difference between the concentrator and subscribers to the central office system, however. The central office unit provides an interface between the overall system and the subscribers and, in essence, is a second order switching unit for establishing appropriate line connections between the central office and the subscribers.
Voice connection between the central office unit is established through a plurality of trunks which extend to a remote switching unit to which a greater number of subscribers are connected. Connections between the subscribers and the control office thus are "concentrated" in the lesser number of trunks.
A feature of the concentrator is that as additional subscribers are added to the system, additional trunks are not necessarily needed, thus lowering installation costs. The line concentrator therefore is particularly adaptable to areas of increasing population growth, such as housing developments.
Since the fundamental purpose of a line concentrator is to concentrate traffic in a small number of trunks, effective utilization is predicated upon a statistical analysis and the resulting assumptions that only a certain number of subscribers will have need for a telephone service at any particular instant in time. It naturally follows that since each subscriber does not have a trunk necessarily allocated to him, the possibility, albeit remote, for a system overload is ever present.
An innovation to line concentrator technology, to minimize the overload problem, is the inclusion of intracall capability. This envisions a line concentrator system, as outlined, but wherein connections are made in the remote field unit between called and calling subscriber so that there is no need to maintain the connection between the two subscribers over the two trunks otherwise needed, thereby freeing these trunks for incoming and outgoing calls and thus increasing the overall traffic handling capability of the system. Thus, in essence, a line concentrator system with intracall capability uses the trunk lines for establishing the initial connection between the called and calling subscriber as well as for connections to the subscribers from the central office for outgoing calls, while the intracall lines are used to maintain the call between the subscribers. A leading patent in this area is U.S. Pat. No. 3,099,717.